Using apple snails as a biological control agent for
parasites
Apple snails (ampullariids) are usable as biological control agents for other
Bulinus and Biomphalaria snails. These pulmonate snails are an
intermediate host for trematoda parasites that can affect man and cause swimmers
itch and schistomiasis. Although apple snails do get infected, the parasite
isn't able to complete its life cycle in most apple snails and no infectious
larvea are released from the snails. However, several reports
have revealed that apple snails can carry and release non-human schistosome
trematodes. In such cases, the cercariae penetrate the skin, where they die
and cause a hypersensitive reaction, resulting in eruptions and intense itching.
This is better known as swimmers-itch.

The actual control of the parasite bearing snails is reached by two means:
Competition and predation.
The apple snails consume most of the available food with their voracious appetite
for vegetation. This in combination with the efficient energy - reproduction
rate of the apple snails, enables them to displace most other snails from their
habitat. Besides the fact that competition element, apple snails also eat the
eggs of other snails and can even predate on other snails directly. Small apple
snail species as Marisa cornuarietis attack their victim
by putting their proboscis (head/mouth) in the shell opening and eating
the snail out, while larger apple snail species are known to destroy the shell
of the other snail in order to reach the soft tissues.

The first time apple snails are used as biological control agent was in the
early 1950's. Marisa cornuarietis
was introduced in Puerto Rico in an experiment to control the Biomphalaria
glabrata population. In the following years a great reduction in biomphalarid
snails was observed and the Marisa snails were observed to feed on the
Biomphalaria glabrata and their eggs.

Similar results are reported from the Dominican Republic and Tanzania. On both
cases, the schistosome bearing snail population was greatly reduced or even
displaced completely by the Marisa snails. Despite these successes, there
is some concern that the widespread of Marisa cornuarietis
could damage rice and taro production.

Other apple snails that have been reported to be successful in reducing Biomphalaria
glabrata, Biomphalaria sudanica and Bulinus globosus are Pila
ovata and Lanistes carinatus. Both
snails species appeared to consume both biomphalarid eggs masses as well the
snails itself, even when lettuce was available ad-libido. In a comparative studie
with Pila ovata, Lanistes carinatus and Marisa cornuarietis,
Pila ovata was reported to be the most successful predator on biomphalarid
snails.

Furhermore Saulea vitrea is known to eat eggs
masses and also adults of schistosome-transmitting snails under laboratorium
conditions.
In Sierra Leone, Saulea snails, are reported not to co-exist with schistosome-transmitting
pulmonates, suggesting a strong competetion, leaving no room for those pulmonates.
Note: temperature can also benefit one species above another. Pulmonates in
Kenia, for example, inhabit the colder areas, whereas ampullariids are more
successful under higher temperatures.

Pomaceaglauca,
Pomacea Haustrum and Pomacea
canaliculata are other apple snail species that have been successfully
used as biological control agents. These snails compete with other snails for
food and predate on them as well.

Despite the fact that many apple snail species successful predate and compete
with schistosome-transmitting snails, cautions should be taken with introducing
exotic ampullariids. There are, unfortunately, enough examples of the devastating
effects that apple snails can have on human food production, as is the case
with the introduction of Pomacea canaliculata in the rice fields of Asia.
Before introducing exotic apple snails species, it should be emphasised that
there are often local ampullariid species that are reported to have an impact
on schistosome-bearing snail populations and that these native ampullariids
should be studied first before exotics are introduced. In case of Africa, Pila,
Lanistes and Saulea snails could be as usefull as the South-American
Marisa and Pomacea snails. One should, however, realise that many
ampullariids often co-exist with schistosome-transmitting pulmonates and that
the success of introducing ampullariids could be temporary.